1. Field of the Invention
This invention relates generally to a well perforating gun and method of perforating a subterranean surface within a well bore, and more particularly, to a perforating gun and method whereby three distinct helically distributed shot patterns can be selectively utilized with a single tool.
2. Description of the Prior Art
A variety of perforating guns and methods have been developed and used heretofore for forming openings known as perforations in a well which extend from the well bore into an adjacent formation. Generally, the perforating gun is loaded with explosive charges and lowered in the well bore on a tubing string or a wireline to a position adjacent the subterranean formation. The perforating gun is usually of an elongated tubular configuration, and the charges are shaped charges having conically constrained explosive material therein which are distributed over the length of the gun. The detonation of the charges can be activated electronically or mechanically depending upon the technique used for lowering the perforating gun in the well bore. When the charges are detonated, elongated explosive jets are formed which produce perforations through any casing, liner, cement, or the like in the well bore and into the subterranean formation adjacent the well bore.
Perforating guns have heretofore been comprised of a closed fluid-tight tubular gun body adapted to be lowered in a well bore having a charge holder disposed therein with explosive charges attached thereto in a selected spatial distribution. Detonating means connected to the explosive charges are activated from the surface when the perforating gun has been positioned adjacent a subterranean formation to be perforated. The explosive jets produced upon detonation of the explosive charges must first penetrate the perforating gun body before penetrating materials in the well bore and the adjacent formation. In order to reduce the resistance produced by the perforating gun body and increase the depth of perforation penetration in the formation, the perforating gun body has heretofore been provided with external recesses which leave relatively thin wall portions through which the explosive jets pass. The recesses in the gun body must be positioned in a spatial distribution thereon which corresponds to the spatial distribution of the explosive charges held within the gun body by the charge holder.
As will be understood, the spatial distribution of the explosive charges held within the perforating gun body by the charge holder and the corresponding spatial distribution of the recesses in the perforating gun body dictate the spatial distribution of the perforations formed by the perforating gun (referred to as the "shot pattern"). Different shot patterns are required to achieve desired results in different well applications. For example, in perforating very thin formations known as "laminated formations", the longitudinal spacing between perforations must be very close. In the perforation of very thick formations, the opposite is true. In deviated well bores and well bores traversing unconsolidated formations, it is often desirable to have increased longitudinal spacing between perforations to prevent the formation from collapsing into the formed perforations and/or into the well bore. The spatial distribution of the explosive charges also affects the shock load applied to the perforating gun structure upon the detonation of the charges. Generally, it has been found in the art that the least shock load is applied to a perforating gun having single charges in horizontal planes which are spaced one below the other longitudinally and which are successively detonated. In order to produce a 360.degree. shot pattern, the charges are circumferentially offset, i.e., the discharge end of each successive charge is laterally spaced from a preceding charge whereby the discharge ends of the charges as well as the corresponding recesses in the perforating gun body lie on a helical path. The successive detonation of the charges along the helical path produces a shot pattern and perforations around and over the length of the perforated interval.
In order to provide perforating guns capable of producing different helical shot patterns in different well applications, it has heretofore been necessary to have available a number of different recessed perforating gun bodies and charge holders. Thus, although there are perforating guns which perform satisfactorily in particular uses, there is a need for an improved single perforating gun design whereby such gun can be utilized in a plurality of different well perforation applications.